Process of lithographic printing



3,15 L58 Patented Dec. 15, 1964 United States Patent 655cc 3,161,508PROQESS F LITHUGRAPHEC PRINTING Martin Hepher, Anthony Marriage, AlanMaurice Vincent, and Leslie Alfred Williams, all of Harrow, England,assignors to Eastman Kodak Lompany, Rochester,

N.Y., a corporation of New Jersey No Drawing. Filed Aug. 26, 1959, Ser.No. 836,977 Claims priority, application Great Britain Aug. 29, 1958 11Claims. (Cl. 96-29) The present invention relates to lithographicprocesses and to lithographic printing plates made photographically.

In lithography a greasy ink is employed and the printing plate comprisesa support which when wet is oleophobic and carries a line or dot imagewhich is oleophilic.

The oleophilic properties of the image and the elecphobic andhydrophilic properties of the surrounding support surface are relativein nature to some extent. When using an image bearing plate forprinting, both ink and water are applied and the support is sufficientlyhydrophilic to acceptwater but not the geasy ink, while the imageaccepts the ink but not the water. Hence to obtain good prints it isnecessary that the diiference in hydrophilic and oleophilic propertiesof the image and v the background surface is sufiiciently great thatwhen water and ink are applied, the image will accept sufiicient inkwithout the background accepting any ink at all. Hence lithographicprinting plates are supports the nature of whose surface is such thatwhen wetted it will not accept the ink although if ink is applied whenthe surface is dry the ink so applied is not removed by subsequentwetting.

The practice inmaking a photolithographic printing plate is to producean oleophilic image on the hydrophilic support which can be put into aninking machine whereby large numbers of copies can usually be made veryrapidly from the one plate.

Where only a few hundreds of copies are required,

or even up toabout a thousand copies, it is quite common to use a papertype. printing plate, but where many thousands of copies are required itis necessary to use i a more substantial type of printing plate and theones commonly used are made of aluminium sheet or zinc sheet. From thesemetal sheets it is a common practice in the art to take as many astwenty thousand copies. Aluminium sheet employed in the art is providedwith an inert hydrophilic surface. This is often done by treating thealuminium to produce thereon an aluminium compound which is hydrophilicfor instance aluminium oxide as in the case of anodised aluminium,aluminium silicate and chromatiscd aluminium.

When making such printing plates, usually an ordinary negative is firstmade and then this is used for making a positive oleophilic image on theprinting plate. A large number or" processes have been used andsuggested for doing this.

A printing process employing greasy ink on metal plates was suggestedmany years ago by K. W. C.Webb in which process, for instance thepolished copper plate, were not as such suitable asllithographicprinting plates and it was necessary to etch the bare copper parts aftertheimage a had been formed. One object of the present invention is toprovide a process which entirely avoids etching after formation of theimage by making the image areas very oleophilic compared with thebackground areas.

In view of the wide variety of commercial uses of photolithographiccopying, it is very desirable to have a process which is both simple andeasy for the user.

An object of the present invention is to provide a process whichsatisfies these requirements in a verysurprisingly simple way namely byforming a silver image on a lithographic printing plate and merelytreating it with a special type of organic compound which is adsorbed tothe surface of the silver and renders it oleophilic while having noappreciable elfect on the hydrophilic proper-,

ties of the background. This treatment of the silver image with thesolution can be very easily done by swabbing or rubbing a solution ofthe organic compound over the image for a few seconds using cotton wool.An important advantage is that there is nothing critical about thistreatment. It is believed that the organic compound is rapidly adsorbedto the surface of the silver and this takes place immediately thesolution is applied thus rendering the time of application very shortand not critical.

Accordingly the present invention provides a'lithographic printingprocess which comprises forming on a lithographic printing surface aphotographic silver image and bath ng the silver image in a solution ofa sulphur or selenium organic compound containing an oleophilic group inthe molecule and being a compound which when brought into contact withthe silver at ordinary room temperature becomes integrated to thesurface of, and only in the region of the silver and thereby renders thesurface of the silver oleophilic by virtue of the oleophilic groups,inking the silver image with greasy printing ink and printing therefromin a lithographic printing press. Preferably the support is free ofmetals and metal salts which are readily converted into oleophilic metalderivatives of said organic compound. By integrated only to the surfacewe mean that the compound is adsorbed to the surface of the silver butdoes not chemically react with the bulk of the silver. Whether theadsorption of a chemical to the surface of a metal is purely a physicalphenomenon is open to doubt since it is believed that surface adsorptionof this kind may involve some surface chemical action of some kind whichis not thoroughly understood. Y

The development of the diffusing silver halide must be sufiicientlygreatto ensure that the developed image on the printing surface is builtup to a suit-able high density at the surface.

In a preferred embodiment there is provided a lithographic printingprocess which comprises forming on a lithographic printing surfacecontaining a silver precipitating agent a silver image from thediffusing silver halide of an exposed and developed silver halideemulsion layer in contact with the printing surface and then bathing thesilver image so formed in a solution of an organic compound containingan oleophilic group in the molecule and being a compound which whenbrought into contact with the silver at ordinary room temperaturebecomes integrated to the surface of, and only in the region of'thesilver and thereby renders the surface of the silver oleophilic byvirtue of the oleophilic groups, inking the silver a image with greasyprinting ink and printing therefrom in a lithographic printing press.

In order that the organic compound shall be adsorbed to the surface ofthe silver it must contain in the molecule one or more groups having anaflinity for silver. Such groups include the following:

(2) --SR where R is a group which can easily be hydrolyzed off leavingSH, such as acoyl, aroyl, amidinium or NHR when R is alkyl, aryl,aralkyl, cyclo alkyl, heterocyclyl.

(3) Thio-acid groups.

(4) Thio-amide groups.

(5 Selenium analogues of the foregoing groups (1), (2),

(3) and (4).

(6) Isothiocyanate group.

In order that the organic compound shall confer oleophilic properties onthe silver, it must contain one or more oleophilic groups in themolecule. In general it is found that suitable oleophiliccharacteristics are conferred on the silver image if the organiccompound contains a. fatty chain or a substituent containing at leastfour carbon atoms, e.g., potassium butyl xanthate.

In some cases however where there are no additional groups in themolecule that detract from its hydrophobic properties the fatty chainmay be one of less than four carbon atoms, e.g., S-methyl isothiuroniumsalts, ethyl isothiuronium salts which split in alkaline solutions togive S-methyl mercaptan and ethyl mercaptan respectively, triazolemercaptans and tetrazole mercaptans.

Organic compounds which contain the aforesaid groups in the molecule andtherefore may be used in the present invention include the following:

(1) Dodecyl mercaptan, benzothiazole-Z-thiol, and the triazine thiols ofthe general Formula I wherein R is an alkyl or cycloalkane, preferablyof, but not limited to, the range C to C These compounds have goodstability when dissolved or dispersed in water or aqueous solutions ofgum arabic or carboxy methyl cellulose.

R is H2 H FORMULA 1 Compounds of this class can be prepared by themethod of or analogous to the method described by Burke J.A.C.S., 69,236-7 (1947). Examples of triazine thiols arel-dodecyl-l,2,4,5-tetrahydro-1,3,5-triazine-4thiol andl-octyl-1,2,4,5-tetrahydro-1,3,5-triazine-4-thiol.

(2) S-diethylamino ethyl isothiuronium chloride hydrochloride.

(3) Thio acids and their salts such as thio-benzoic acid or potassiumoctylxanthate, dithiocarbamates, for exam ple diethyldithiocarbamate,zinc dibutyldithiocarbamate, the polysulphide derivatives ofdithiocarbamates which are activated by alkali, e.g.,tetraethylthiuramdisulphide.

(4) Thio amides, e.g., phenylthiourea, N-(N-octylthiocarbamoyl) glycine,thiopropionanilide, phenylthiosemicarbazide.

(5) The selenium analogues of the above, e.g., phenylselenosemicarbazide, 2-selenylbenzothiazole.

(6) Isothiocyanates, e.g., phenylisothiocyanate.

The silver precipitating agents such as development nuclei on thelithographic printing support should preferably be free of particles ofcompounds such as zinc sulphide because many of the said organiccompounds having the properties required for making the surface of thesilver oleophilic chemically react with zinc sulphide to convert it intothe metal derivative of said organic compound, and we have found thatsuch conversion of the zinc sulphide nuclei in the background rendersthose nuclei sufficiently oleophilic to cause the background to acceptvisible amounts of ink unless the plate is wetted extensively whenmaking the copies therefrom. On the other hand it is believed than anysurface action, such as adsorption by metal or metal salt nuclei is notpractically detrimental and for this reason nuclei of for instancesilver or silver sulphide, which are not converted into particles of asilver salt of the organic compound are the preferred kind of nuclei toemploy in this invention. If a low concentration of zinc sulphide nucleiis used in order to try to avoid this difficulty caused by theconversion, there are insutficient nuclei to yield a sufficiently strongsurface image.

When, as is preferred, the development nuclei comprises silver or silversulphide it is necessary to use as little colloidal binder as possiblefor holding the nuclei on the surface of the support since if too muchbinder is used, the silver image which is formed is protected by thebinder and is not sufficiently strong at the surface to operatesatisfactorily in the linking process. It is preferable to obtainadherence between the nuclei and the surface by means other than using acolloidal binder. We have found that an excellent support to use in ourinvention is aluminum the surface of which comprises a hydrophilicaluminum compound, for example we prefer anodised aluminum. Othersupports may be used in the present invention for example, grained zincchemically treated to produce a hydrophilic lithographic surface, andpaper lithographic supports. In the case of paper supports it ispreferable to render the paper waterproof or of low water permeabilityand to apply to the surface a thin hydrophilic layer, such as a layer ofcasein or silica or polyvinyl alcohol with suitable fillers if necessaryand then silver nuclei may be deposited thereon and will adhere thereto.Various plastics may also be used as supports, for example surfacehydrolyzed cellulose ester sheetings.

A suitable nucleated surface can be produced on aluminium which has beentreated to convert the surface into a hydrophilic aluminium compound foruse in making printing plates by the present invention by merely bathingthe surface of the aluminium with a colloidal solution of silver or goldor silver sulphide. An alternative but less convenient way of formingsuch nuclei on the aluminium surface is to bathe the aluminium surfacefirst with a solution of silver nitrate followed by bathing with sodiumsulphide thus producing nuclei of silver sulphide.

If the nucleating layer is formed by simply bathing the anodisedaluminium in a simple aqueous colloidal solution of silver this ensuresthat the amount of the silver which is absorbed by the anodisedaluminium surface per unit area is extremely low and no colloid binderis necessary. Colloidal solutions of silver which may be used for thispurpose are, for instance, those giving yellow, red or black solutionsand containing between 0.1 and 0.001 mol of silver per litre.

The simplest way of treating the silver image with the organic compoundaccording to our invention is to form the silver image first and thenmerely apply to it a solution of the organic compound. However, thesilver image may be formed in the presence of the said organic compound.

We have also discovered that the image can be treated with the organiccompound by coating the organic compound on the aluminium plate beforeforming the image thereon and in this case the organic compound becomesadsorbed to the silver as the image is formed. However, in this case itis believed that adsorption of the organic compound by the silver imagedepends on whether the support has been specially coated withdevelopment nuclei, such as colloidal silver or silver sulphideparticles and the concentration of such nuclei. If the concentration ofnuclei is high then a black image of metallic silver is obtained withthe organic compound adsorbed or partly adsorbed thereto. However if theconcentration of nuclei is very low or nil a whitish image may resultwhich comprises largely or wholly the silver derivative of the saidorganic compound formed by double decomposition of the silver halidewith the organic compound. 7

It is also possible to produce an oleophilic image if the said organiccompound is present in the developing solution when the silver image isformed by the silver salt diffusion process and both the receivingsupport and the silver halide emulsion layer are immersed therein duringthe formation of the image.

If desired a two layer material may be usedin the present inventionnamely one in' which the receiving layer (which may contain developmentnuclei) is coated beneath the light sensitive silver halide layer itonly being necessary to remove, such as by washing away, the latterlayer after the silver image has been formed in the receiving layer. Inthis .case the emulsion layer may be for instance, a cellulose dib'asicacid ester emulsion layer described in the Yackel et al., US. Serial No.586,705, filed May 23, 1956.

Example 1 A silver sol is prepared by the classical Carey Lea method andis Well Washed and finally diluted to 1 gram in 100 cos. of distilledwater. A sheet of aluminium 0.005 inch thickand anodised to give atihckness of aluminium oxide of about 0.5 gram per square metre isbathed in this solution, the excess removed and the sheet dried.

A high contrast negative material such as Kodak Chemical TransferNegative Paper comprising a sensitive silver chloride emulsion isexposed either by transmission or by reflex and in conjunction with theprepared'metal receiving sheet is fedthrough a normal processing machinefor the silver diffusion process such as the Remington Rand TC; 35,containing a developer of the following composition:

7 Grams Sodium hydroxide 11.25 Sodium sulphite (anhydrous) 75.00Hydroquinone 18.0 1-phenyl-3-pyrazolidone 1.0 Potassium bromide 0.25Hypo 3.5

Water to 1 litre.

The two sheets are squeegeed together as they emerge from the developerand are left in contact for /2 minute after which they are strippedapart and the transferred silver image on the aluminium receiving sheetis rinsed and then is swabbed for a few seconds with the followingsolution:

l octyl 1,2,4,5 tetrahydro 1,3,5 triazine- 4-thiol grams 0.5

Acetone ccs 50 Water do 50 The excess is removed'with a sponge moistenedwith a weak solution of gumarabic after which the image may be inked inthe usual way either by hand or on a lithographic printing machine, andwill give several thousands of copies.

Further examples of solutions which may be used for treating the silverimage in the foregoing are: (a) 1 dodecyl vl,2,4,5 tetrahydro 1,3,5tritetrah.ydrol,3,5 -triazine-4thiol in acetone is shaken vigorouslywith 50 ml. of a 5% aqueous. solution of carboxy, methyl cellulose toform a crude emulsion. i 0

Water .j.... ccs 100 The colour of theimage is hardly affected at all bytreatment with any of the solutions described with reference to theforegoing example. j

Further examples of nucleated supports which may be used in the presentinvention are:

Example 2 1 gram of chloroauric acid H(AuCl is dissolved in 500 ml. ofdistilled water, it is neutralized with 0.1 N potassium carbonate andheated. A few drops of a solution of White phosphorus in ether are addedto produce a gold sol. This gold sol. is washed and diluted to 1 litreand a sheet of anodised aluminium bathed in it and dried.

- E ple 3 This is the same as Example 2v except that the metal sheetinstead of being anodised aluminium is a sheet of aluminium on thesurface of which a fine sharp lithographic grain has been produced by achemical etching technique to produce for instance an aluminum silicateor chromate surface, a suitable material being that sold commercially byAddressograph Multigraph for conventional lithographic platemaking.

Examples of further solutions which may be used in the foregoing Example1 where somewhat less strong oleophilic characteristics in the treatedsilver image are desired or can be tolerated are: i (d) Ethyl'iso-thiourea grams Sodium hydroxide do 1 Water ccs 30 Ethanol ccs 70(e) Dipentamethylene thiuram tetrasulphide grams; 0.25 Sodium hydroxidedo 1.0 i Water ccs 30 Ethanol L ccs 70 (f) 2-seleny1 benzothiazole grams1 Water ccs 30 Ethanol ccs 70 (g) Phenyl selenosemicarbazide grams 1Water ccs 30 Ethanol ocs 70 (h) Zinc dibutyldithiocarbamate ?ms 1 Waterccs 30 Ethanol ccs 70 (i) S-diethylaminoethyl iso thiuronium chloridehydrochloride grams 1 Sodium hydroxide do 1 Water Q ccs 100 While asstated above it isvery desirable to have the background free of metalsand metal salts which are readily converted in metal derivatives of theorganic compound used to treat the silver, we have found that in thecase of some paper supports which have on the surface -a much thickerhydrophilic layerxthan is the case with anodised aluminium and'the like,it is possible to use zinc sulphide nuclei and then by keeping theprinting plate very well wetted during printing several dozen goodcopies I can be obtained, as in the following example.

Example 4 A lithographic paper plate sold by the AddressographMultigraph Corporation under the name Blue Duplimat is first treatedwith a solution of 1% sodium sulphide in water for about 20 seconds andthen washed for 2 to 3 minutes and dried off. This treatment is believedto form zinc sulphide in the Duplimat'coating by'the reaction with asoluble zincsalt already incorporated during manufacture of thematerial. Such treatment provides a suitable nucleated receiving layer.V

The paper plate is now wiped over with the following solution and'driedoff: Hypo V -gram-.. 2 Sodium carboxy methyl cellulose (high Viscosity)then ink accepting by wiping over with:

Potassium butyl xanthate "grams" 2 Water ccs 100 The colour of the imageis hardly changed at all by this treatment but now readily acceptsgreasy ink while the background refuses the ink if maintained in a wellwetted condition by applying plenty of water in the printing machine.Several hundred copiescan be run off.

An example illustrating the treatment of the silver image byincorporating the organic compound in the developing solution is asfollows:

Example 5 A sheet of negative paper such as Kodak Chemical Transferpaper is exposed as in Example 1 and then together with a sheet ofanodised aluminium carrying silver nuclei is fed through a processingmachine as in Example 1, and using the same developer but alsocontaining 5 grams per litre of1-hexyl-l,2,4,5-tetrahydro-l,3,5-triazine-4-thiol. When the two sheetsare separated the aluminium sheet carries a positive silver image whichis oleophilic. It is wiped over with a 2% solution of gum arable andthen inked in the usual way.

Examples illustrating the application of the organic compound to thereceiving support prior to the formation of the silver image thereon areas follows:

Example 6 A sheet of anodised aluminium foil nucleated by treatment witha silver sol. as in Example 1 is coated with a thin layer of 0.5%solution of 1-octyl-l,2,4,5-tetrahydro- 1,3,5-triazine-4-thiol in ethylalcohol and then dried off.

A sheet of light sensitive halide negative material such as KodakChemical Transfer paper is exposed to a positive image and then inconjunction with the sheet of coated anodised aluminum foil is fedthrough a Remington Rand TC. 35 machine containing the same developer asin Example 1. When the sheets have been separated the aluminum sheetbearsja black positive image. This sheet is then wiped with a swabcontaining ethyl alcohol to remove the unused triazine compound. A 2%solution of gum arabic is then wiped on the aluminium and then it isinked in the usual way.

Example 7 A sheet of Blue Duplimat is treated with sodium sulphide as inExample 4.

The paper plate is now wiped over with a 2% solution of hypo in waterand dried 01f. A sheet of light sensitive silver halide negativematerial such as that supplied for the solvent transfer process, e.g.,Kodak Chemical Transfer paper is exposed to a positive image, developedfor about 20 seconds in the developer used in Example '1 but omittingthe hypo, and then while still containing the developer it waslaminatedinto contact with the paper Duplimat. After a few-seconds thesheets are separated and a positive image is visible on the Duplimatsheet. The surface of this sheet is now moistened with Platex solutionor with a mixture of:

Platex is a proprietory solution sold by Addressograph MultigraphLimited for treating Duplimat sheets.

l i The excess liquid is removed with a damp sponge and this is followedby rubbing over with greasy lithographic ink on a pad or cloth but itwill be seen that it is not possible to get much ink to adhere to thesilver image. Instead of using the above prepared Duplimat another sheetis taken and after being treated with the sodium sulphide solution iswiped over with the following solution and dried 01f:

Hypo grams 2 Potassium butyl xanthate do 1 Water ccs The remainder ofthe process is then carried out as before. It will be found that thetransferred silver image which is almost black in colour readily acceptslithographic ink when applied from the cloth.

Alternative solutions which may be used in this example for coating onto the Duplimat sheet after treatment with the sodium sulphide are:

When these solutions are used the images are again black in colour.

An example Where the support is coated with the organic compound beforethe image is formed thereon and the image is formed by doubledecomposition of the diffusing silver halide with the organic oompound,is as follows:

Example 8 A sheet of brush grained aluminium foil is surface anodized inthe known manner to give an aluminium oxide layer of about 0.5 gram persquare metre and coated with the following solution:

Potassium octyl xanthate grams 1 Hypo do 2 Water ccs 100 An exposednegative paper is developed and then washed and laminated to the treatedaluminium surface and left for about 30 seconds. It is then strippedaway and there is seen on the aluminum surface a whitish positive imageof silver xanthate. The surface is then wiped over with a gum-phosphoricacid mixture and then rinsed off. This image readily accepts greasy ink.

We claim:

1. A lithographic printing process which comprises forming on a supportcontaining a silver precipitating agent a silver image from thediffusing silver halide of an exposed and developed silver halideemulsion layer in contact with the support and then directly bathing thesilver image so formed in a solution of a compound of the classconsisting of oleophilic organic compounds containing a thiol group,oleophilic organic compounds generating a thiol group, and seleniumanalogues of said compounds, inking the silver image with greasyprinting ink and printing therefrom in a lithographic printing press.

2. A lithographic printing process comprising forming on a supporthaving development nuclei coated thereon a silver image by developmentof the diffusing silver halide from an exposed and developed silverhalides layer in contact therewith and directly bathing the silver imageon the support in a solution of a compound of the class consisting ofoleophilic organic compounds containing a thiol group, oleophilicorganic compounds generating a thiol group, and selenium analogues ofsaid compounds, inking the silver image with greasy printing ink andprinting therefrom in a lithographic printing. press.

3. A lithographic printing process according to claim 1 wherein thesupport is free of metals and metal salts which are readily convertedinto metal derivatives of said organic compound.

4. A lithographic printing process according to claim 1 wherein thesupport is aluminium which has been chemically treated to form a surfacelayer of a hydrophilic aluminium compound.

5. A lithographic printing process according to claim 1 wherein thesupport is coated with development nuclei of the class consisting ofsilver, silver sulfide and gold.

6. In a lithographic printing process which comprises forming on anucleated support a photographic silver image by diffusing silver halideimagewise from an exposed and developed silver halide emulsion over thesaid support and removing the said emulsion, the step of directly afterformation of the silver image bathing the image in a solution of acompound of the class consisting of oleophilic organic compoundscontaining a thiol group, oleophilic organic compounds generating athiol group and selenium analogues of,said compounds, inking the silverimage with greasy printing ink and printing therefrom in a lithographicprinting press.

7. A process for preparing'a photographic image from an exposed silverhalide emulsion layer comprising developing the emulsion layer,transferring the silver halide from the undeveloped areas of theemulsion layer to a. support containing a silver precipitating agent inthe presence of a silver halide solvent and directly following theformation of the silver image on the said support, contacting the silverimage with an organic compound containing an oleophilic group in themolecule and selected from the compounds of the general formula whereinR is a member of the class consisting of alkyl and cycloalkane groups,inking the silver image with greasy printing ink and printing therefromin a lithographic printing press.

8. A process for preparing a photographic image from an exposed silverhalide emulsion layer comprising developing the emulsion layer,transferring the silver halide from the undeveloped areas of theemulsion layer to a support containing a silver precipitating agent inthe presence of a silver halide solvent and directly following theformation of the silver image on the said support, contacting the silverimage with an organic mercaptan containing an oleophilic group in themolecule which is 10 capable of rendering the silver image oleophilic,inking the silver image with greasy printing ink and printing therefromin a lithographic printing press.

9. A process for preparing a photographic image from an exposed silverhalide emulsion layer comprising developing the emulsion layer,transferring the silver halide from the undeveloped areas of theemulsion layer to a support containing a silver precipitating agent inthe presence of a silver halide solvent and directly following theformation of the silver image on the said support, contacting the silverimage with a thio-acid containing an oleophilic group in the moleculewhich is capable of rendering the silver image oleophilic, inking thesilver image with greasy printing ink and printing therefrom in alithographic printing press.

10. A process for preparing a photographic image from an exposed silverhalide emulsion layer comprising developing the emulsion layer,transferring the silver halide from the undeveloped areas of theemulsion layer to a support containing a silver precipitating agent inthe presence of a silver halide solvent and directly following theformation of the silver image on the said support, contacting the silverimage with a thio-amide containing an oleophilic group in the moleculewhich is capable of rendering the silver image oleophilic, inking thesilver image with greasy printing ink and printing therefrom in alithographic printing press.

11. A process for preparing a photographic image from an exposed silverhalide emulsion layer comprising developing the emulsion layer,transferring the silver halide from the undeveloped areas of theemulsion layer to a support containing a silver precipitating agent inthe presence of a silver halide solvent and directly following theformation of the silver image on the said support, contacting the silverimage with an isothiocyanate containing an oleophilic group in themolecule which is capable of rendering the silver image oleophilic,inking the silver image with greasy printing ink and printing therefromin a lithographic printing press.

References Cited in the file of this patent UNITED STATES PATENTS2,987,396 Williams et al. June 6, 1961 FOREIGN PATENTS 1,058,844 GermanyJune 4, 1959 783,793 Great Britain Oct. 2, 1957 758,759 Great BritainOct. 10, 1956 556,753 Belgium Oct. 17, 1957 1,011,280 Germany June 27,1957 OTHER REFERENCES Belgium 556,753, abstracted in PhotographicAbstracts, pub. by The Royal Photographic Society of Great Britain,London, vol. 38, part 4, 1958, p. 325.

1. A LITHOGRAPHIC PRINTING PROCESS WHICH COMRPISES FORMING ON A SUPPORTCONTAINING A SILVER PRECIPITATING AGENT A SILVER IMAGE FROM THEDIFFUSING SILVER HALIDE OF AN EXPOSED AND DEVELOPED SILVER HALIDEEMULSION LAYER IN CONTACT WITH THE SUPPORT AND THEN DIRECTLY BATHING THESILVER IMAGE SO FORMED IN A SOLUTION OF A COMPOUND OF THE CLASSCONSISTING OF OLEOPHILIC ORGANIC COMPOUNDS CONTAINING A THIOL GROUP,OLEOPHILIC ORGANIC COMPOUNDS GENERATING A THIOL GROUP, AND SELENIUMANALOGUES OF SAID COMPOUNDS, INKING THE SILVER IMAGE WITH GREASYPRINTING INK AND PRINTING THEREFROM IN A LITHOGRAPHIC PRINTING PRESS.